This invention relates generally to micro-electro-mechanical systems (MEMS). In particular, it provides method and system for making MEMS mirrors by a combination of bulk and surface micromachining techniques.
MEMS mirrors have demonstrated to be effective in a variety of applications, including high-speed scanning and optical switching. In such applications, it is essential for MEMS mirrors to have flat optical surfaces, large rotational range, and robust performance.
Many applications (e.g., optical networking applications) further require that MEMS mirrors be configured in a closely packed array. It is therefore desirable to maximize the xe2x80x9coptical fill factorxe2x80x9d of the array (i.e., by making the optical surface of each constituent mirror as large as possible), without compromising other essential characteristics.
MEMS mirrors are conventionally made by either bulk or surface silicon micromachining techniques. Bulk micromachining, which typically produces single-crystal silicon mirrors, is known to have a number of advantages over surface micromachining, which typically produces polysilicon (or thin-film) mirrors. For example, single-crystal silicon mirrors produced by bulk micromachining techniques are generally thicker and larger mirrors with smoother surfaces and less intrinsic stress than polysilicon (or thin-film) mirrors. Low intrinsic stress and sizeable thickness result in flat mirrors, while smooth surfaces reduce light scattering. An advantage inherent to surface micromachining techniques is that the mirror suspension (e.g., one or more thin-film hinges) can be better defined and therefore made smaller. This allows the MEMS mirror thus produced to have a large rotational range, e.g., at moderate drive voltages.
U.S. Pat. No. 6,028,689 of Michalicek et al. (xe2x80x9cMichalicek et al.xe2x80x9d) discloses a movable micromirror assembly, driven by an electrostatic mechanism. The assembly includes a mirror supported by a plurality of flexure arms situated under the mirror. The flexure arms are in turn mounted on a support post. Because the assembly disclosed by Michalicek et al. is fabricated entirely by way of surface micromachining techniques, the resulting xe2x80x9cmicromirrorxe2x80x9d is of the polysilicon (thin-film) type and is thus subject to the aforementioned disadvantages.
International Patent Application Number WO 01/94253 A2 of Chong et al. discloses a MEMS mirror device having a bulk silicon mirror attached to a frame by thin-film hinges. A notable shortcoming of this system is evident in that the thin-film hinges extend from the reflective surface side of the mirror to the frame, hence restricting (or obstructing) the amount of surface area available for optical beam manipulation. This shortcoming further results in a lower optical fill factor in an array of such MEMS devices.
Tuantranont et al. disclose an array of deflectable mirrors fabricated by a surface micromachining polysilicon (or xe2x80x9cMUMPSxe2x80x9d) process in xe2x80x9cBulk-Etched Micromachined and Flip-Chip Integrated Micromirror Array for Infrared Applications,xe2x80x9d 2000 IEEE/LEOS International Conference on Optical MEMS, 21024, Kauai, Hi. (August 2000). In this case, an array of polysilicon mirror plates is bonded to another array of thermal bimorph actuators by gold posts using the xe2x80x9cflip-chip transfer techniquexe2x80x9d, resulting in trampoline-type polysilicon plates each suspended at its corners by thermal bimorph actuators. In addition to the mirror plates made of polysilicon (or thin-film), another drawback of the thus-constructed mirror array is the lack of a monolithic structure, which makes the array susceptible to misalignment and other extraneous undesirable effects.
In view of the foregoing, there is a need in the art to provide a novel type of MEMS mirrors that overcomes the limitations of prior devices in a simple and robust construction.
The present invention provides a MEMS apparatus, including a bulk element; a support; and one or more hinges. The bulk element comprises a device surface and a bottom surface that is situated below the device surface. The hinges are disposed below the bottom surface of the bulk element and couple the bulk element to the support, whereby the bulk element is suspended from the support.
In the above apparatus, the support may include a cavity, in which the hinges are disposed. There may be at least one electrode disposed in the cavity, for causing the bulk element to be actuated. The device surface of the bulk element may further contain a reflective layer (e.g., a metallic film), rendering the apparatus thus constructed a MEMS mirror.
In the present invention, the term xe2x80x9cbulk elementxe2x80x9d refers to an element fabricated by bulk micromachining techniques known in the art, which typically comprises a single-crystal material. A case in point may be a single-crystal silicon element. The bulk element is characterized by a xe2x80x9cdevicexe2x80x9d surface and a xe2x80x9cbottomxe2x80x9d surface that is situated below the device surface (while the bulk element itself may assume any geometric form deemed suitable). The xe2x80x9cdevicexe2x80x9d surface of the bulk element may be optically reflective. It may also be used as an xe2x80x9cinterfacexe2x80x9d for coupling the bulk element to other devices, if so desired in a practical application. Further, a xe2x80x9csupportxe2x80x9d may be a frame or substrate, to which the bulk element is attached. A xe2x80x9chingexe2x80x9d (or xe2x80x9chinge elementxe2x80x9d) should be construed broadly as any suspension/coupling means that enables the bulk element to be suspended from the support and further provides the restoring force as the bulk element undergoes motion. For instance, a hinge may be a flexure or flexible coupling, e.g., fabricated by a bulk or surface micromachining technique known in the art. The term xe2x80x9cunderneathxe2x80x9d refers to the hinges being anchored to (or below) the bottom surface of the bulk element and thereby disposed wholly beneath the device surface. This allows the device surface of the bulk element to be maximized and the entire surface to be usable (e.g., for optical reflection).
The present invention further provides a process flow (or method) that may be used for fabricating the aforementioned MEMS apparatus. In one embodiment of a process flow according to the present invention, a xe2x80x9cdevicexe2x80x9d component is formed. The device component in one form may be provided by an SOI (Silicon-On-Insulation) wafer, comprising a single-crystal silicon device layer and a silicon handle wafer with an insulation layer (e.g., silicon oxide) sandwiched in between. First and second hinge elements may be fabricated on a first surface of the single-crystal silicon layer, e.g., by way of surface micromachining techniques. A xe2x80x9csupportxe2x80x9d component is configured to contain a cavity, in which at least one electrode may be disposed. Subsequently, the device and support components are bonded in such a manner that the hinge elements are disposed within the cavity. The silicon handle wafer along with the insulation layer in the device component is then removed, thereby revealing a second surface of the single-crystal silicon device layer. A bulk element may be subsequently produced in the single-crystal silicon device layer by way of bulk micromachining techniques, characterized by the first and second surfaces. The configuration may be such that the hinge elements are each anchored to the first (or xe2x80x9cbottomxe2x80x9d) surface of the bulk element on one end and to the support component on the other, thereby enabling the bulk element to be suspended with the hinge elements wholly underneath the second (or xe2x80x9cdevicexe2x80x9d) surface. A reflective layer may be further deposited on the device surface of the bulk element, rendering the apparatus thus constructed a MEMS mirror.
One advantage of the MEMS apparatus of the present invention is that by placing the hinge elements underneath the bulk element, the device surface of the bulk element can be maximized and the entire surface becomes usable (e.g., for optical beam manipulation). Such a feature would be highly advantageous in making arrayed MEMS devices, such as an array of MEMS mirrors with a high optical fill factor. Further, by advantageously making use of both bulk and surface micromachining techniques, a MEMS mirror of the present invention is equipped with a large and flat mirror along with flexible hinges, and is hence capable of achieving a substantial rotational range at moderate electrostatic drive voltages. An additional advantage of the MEMS apparatus of the present invention is evident in its monolithic structure, rendering it robust in performance. These advantageous features are in notable contrast with the prior devices described above.
The novel features of this invention, as well as the invention itself, will be best understood from the following drawings and detailed description.